Chemical Matting in Powder Coatings: How Reactive Matting Hardeners Work
Category: Powder Coatings
The principle of chemical matting, how it differs from physical matting with waxes and fillers, and the key application points for a stable, reproducible matte finish.
1. Why Matte Finishes Need a Dedicated Matting Technology
Matte and low-gloss powder coatings are widely used on furniture, architectural profiles, appliances and machinery, where a soft, glare-free surface is preferred. Reducing the gloss of a cured powder coating film is not simply a matter of adding a "matting powder": the matting route chosen determines how low the gloss can go, how stable it remains from batch to batch, and how much the leveling and mechanical properties of the film are affected.
Two routes are used in practice: physical matting, which relies on non-reactive waxes or fillers, and chemical matting, which uses a reactive matting hardener that takes part in the curing reaction itself. This article explains how chemical matting works, where it outperforms physical matting, and what to watch in formulation and processing.
2. The Principle: Dual Reaction Rates Create Uneven Shrinkage
Taking a reactive matting hardener of the MB68 type as an example, its curing reaction with epoxy resin proceeds through two parallel processes: (a) the imidazoline component reacts directly with the epoxy groups, and (b) the hardener catalyzes the homopolymerization of the epoxy resin. Because the functional groups in the molecule react with the resin at different rates, the film shrinks unevenly as it cures and a micro-rough surface is formed.
A direct consequence of this mechanism is that the optimum hardener ratio cannot be derived from a stoichiometric calculation. Because two reactions compete, the best ratio for a target gloss is determined empirically, by a dosage trial series in the actual resin system.
3. Chemical Matting vs. Physical Matting
The table below compares the two routes on the points that matter most in production:
| Aspect | Chemical Matting (Reactive Hardener) | Physical Matting (Wax / Filler) |
|---|---|---|
| Principle | Reactive: the hardener joins the curing reaction; reaction-rate differences and uneven shrinkage roughen the surface microscopically | Non-reactive: waxes, metal soaps or fillers such as silica form a micro-rough layer or scatter light at the surface |
| Achievable gloss (60°) | Can reach below 10 in pure epoxy systems; adjustable over a wide range | Wax-based systems are difficult to bring below 30% gloss; filler-based systems cannot practically reach very low gloss |
| Leveling & mechanical properties | Leveling and mechanical properties are largely preserved; film surface stays smooth and fine | High filler loadings noticeably reduce leveling and mechanical properties; excess wax can exude and cause surface haze |
| Gloss stability | Good gloss reproducibility once the ratio and curing window are fixed | Wax distribution is sensitive to extrusion conditions, so gloss tends to fluctuate |
| Typical drawbacks | Requires strict extrusion temperature control to avoid pre-reaction | Wax can impair recoating and intercoat adhesion; scratch resistance may suffer |
In short: for dead-matte and low-gloss finishes with good leveling — especially gloss below 30% — chemical matting is the more reliable route, while physical matting remains a simple option for moderate gloss reduction where its side effects are acceptable.
4. MB68: A Chemical Matting Hardener for Epoxy and Hybrid Systems
MB68 is a representative chemical matting hardener. Chemically, MB68 is the 1:1 mono-salt of pyromellitic acid and 2-phenylimidazoline (CAS No. 54553-90-1, melting range 210–230 °C), the equivalent of the matting hardener formerly sold as Degussa (now Evonik) VESTAGON B 68. MB68 acts as a curing agent as well as a matting agent: it crosslinks the epoxy resin while its dual reaction paths generate the matte effect, so no separate matting additive is needed.
In hybrid (epoxy–polyester) systems, gloss is adjusted by substituting MB68 for part of the polyester while simultaneously increasing the epoxy content, which moves the 60° gloss across a range of about 10–70. Product details and technical data are available on the MB68 product page.
5. Application Guide
- Dosage: Use 8–10 parts of MB68 per 100 parts of E-12 epoxy resin as the empirical starting point; this typically corresponds to 3–5% of the total formulation.
- Gloss targets: Pure epoxy coatings formulated with MB68 can reach a 60° gloss below 10. Hybrid coatings can be adjusted between 10 and 70 by the substitution method described above.
- Extrusion: Extrude at 95–105 °C and never above 110 °C. Repeated extrusion must be avoided, because pre-reaction before spraying destabilizes both gloss and leveling.
- Curing: The curing reaction starts above 180 °C; typical schedules are 180 °C for 20–25 min, 190 °C for 12–15 min or 200 °C for 10–12 min.
- Grinding: After cooling and flaking, grind and sieve the powder (180 mesh) as usual; no special equipment is required.
6. Common Problems and Fixes
Gloss fluctuation between batches
The most common causes are an extrusion temperature that is too high (above 110 °C) or repeated extrusion, both of which trigger pre-reaction, and curing conditions that drift outside the curing window. The fix is procedural: control the extrusion temperature strictly, keep one fixed curing schedule, and lock the hardener ratio after a small-scale trial.
Orange peel and poor leveling
In chemically matted systems, make sure the formulation still contains enough leveling agent and a degassing agent such as benzoin; the matting hardener itself does not replace them. In filler-matted systems, orange peel is usually a sign that the filler loading is too high.
7. Frequently Asked Questions
Q1: What is the difference between chemical matting and physical matting in powder coatings?
Chemical matting uses a reactive hardener that takes part in the curing reaction; the difference in reaction rates causes uneven shrinkage and a microscopically rough surface that scatters light. Physical matting relies on non-reactive waxes or fillers to roughen the surface; waxes can hardly produce gloss below 30%, and high filler loadings harm leveling and mechanical properties.
Q2: How much MB68 should be used in an epoxy powder coating?
A practical starting point is 8–10 parts of MB68 per 100 parts of E-12 epoxy resin, typically 3–5% of the total formulation. Pure epoxy coatings can reach a 60° gloss below 10, and hybrid systems can be adjusted between 10 and 70. The optimum ratio is set by trial, not by stoichiometric calculation.
Q3: Why does the gloss of a chemically matted coating sometimes fluctuate?
The most common causes are an extrusion temperature above 110 °C or repeated extrusion, which trigger pre-reaction, and curing conditions outside the curing window — the curing reaction of MB68 starts above 180 °C. Extruding at 95–105 °C with a fixed curing schedule gives the most reproducible gloss.